Field of the Invention: This invention relates to devices for detecting changes in intrauterine pressure during labor, and specifically relates to catheter devices for determining pressure changes in the uterus through transference of uterine pressure to a gas-containing compliant chamber associated with a closed air column.
Statement of the Art: It has become common practice in the delivery and birth of a baby to monitor intrauterine conditions throughout the period of labor. Monitoring the intrauterine environment, including fetal heart rate and uterine contractions, enables the attending medical personnel to evaluate the progress of the delivery and to diagnose the existence of, or potential for, emergency situations which require immediate attention or action. Where intrauterine monitoring was once of singular importance in high risk and difficult deliveries, it has now become a routine part of the delivery procedure in many births. While only ten percent of all births are considered to be high-risk, warranting the use of intrauterine monitoring, intrauterine monitoring is used in sixty to seventy percent of all births regardless of the level of risk attributed to the birth. In actuality, about twenty percent of births experience complications.
It has long been recognized that an important relationship exists between fetal heart rate (FHR) and intrauterine pressure (IUP) and that such data relates to the well-being of the fetus during labor and delivery. Historically, two types of uterine monitoring have been practiced--external and internal. External uterine monitoring essentially comprises the attachment of a monitoring device to the mother's abdomen. The external uterine monitoring device generally detects fetal heart rate, and may have some capability for detecting intrauterine pressure or other indicia of the labor process. However, external monitoring is limited in its effectiveness because of the inaccuracy of readings obtained from the device. As uterine contractions increase, more "noise" is detected in such systems and data output becomes difficult to interpret. In addition, externally attached uterine monitoring devices move when the patient moves and, therefore, require frequent repositioning.
Known internal uterine monitoring systems, or "intrauterine" devices, include fetal scalp electrodes and pressure sensors positioned within the uterus. Internal monitoring systems are more accurate than external monitoring devices because they detect intrauterine conditions directly and thereby avoid the inaccuracy introduced by noise and other detection artifacts experienced with external uterine monitoring devices.
A number of intrauterine monitoring devices have been disclosed in the patent literature, including U.S. Pat. No. 4,785,822 to Wallace; U.S. Pat. No. 4,966,161 to Wallace, et al.; and U.S. Pat. No. 5,279,308 to DiSabito, et al. Those and other intrauterine pressure monitors employ one of two basic types of pressure detection. One type of pressure detection employs a fluid-filled tube which translates a mechanical change in fluid level within the tube to an electrical signal. A second type of pressure detection employs an electronic sensor positioned near the distal end of the device inserted in the uterus. Both types of intrauterine pressure detection devices detect timing and magnitude of changes in pressure within the uterus. Such changes within the uterus are indicative of phases of uterine contraction, and relate to the well-being and status of the fetus during labor and delivery.
While intrauterine monitoring devices are more preferred than external uterine monitoring devices for the reasons given above, the performance of fluid-filled IUP devices can be problematic because the tubing of such devices can become clogged with particulate matter from the uterus or amniotic fluid. As a result, flushing and recalibration of fluid-filled IUP devices are frequently necessary. While flushing and recalibration are not difficult in such devices, the sterility of the device may be severely compromised and interruption of the monitoring procedure is inconvenient. In addition, fluid-filled IUP devices require refilling and recalibration if the patient is ambulatory. Therefore, use of such fluid-filled IUP devices may limit the ability of the patient to move or walk around during long periods of labor.
Recognizing the drawbacks and inherent problems associated with fluid-filled IUP devices, others have developed gas column pressure catheters for use in monitoring pressures in vessels and cavities of the human body. A catheter of such type is disclosed in U.S. Pat. No. 5,573,007 to Bobo. Gas column devices of the type disclosed in the pending application to Bobo are suitable for use in vessels or cavities where little or no impact may be encountered by the gas-filled chamber of the device. However, in applications where the gas-filled chamber is likely to encounter movement (e.g., the movement of a fetus in utero) or be crushed by other means such as bending of an artery or vein as the patient moves, the gas-filled chamber is crushed or deflated and proper pressure monitoring cannot be conducted. That problem has been addressed in some designs by providing dual gas-filled chambers for pressure monitoring so that if one is crushed or otherwise rendered inoperable because of surrounding conditions, presumably the other gas-filled chamber will operate.
Sensor-tipped IUP devices are the more recently developed of the IUP devices and have gained great popularity over fluid-filled devices because of the relative convenience in use. Minimal setup procedures are required with the sensor-tipped devices other than "zeroing" the device. Zeroing the device involves setting the base pressure to match that of the atmosphere while no pressure is being applied to the catheter. Once inserted, the catheter is connected to a fetal monitor using a reusable interface cable in the same way that the remote sensor of a fluid-filled catheter is connected to a fetal monitor.
Despite the advantages that sensor-tipped IUP devices present over fluid-filled IUP devices, known sensor-tipped devices still have certain disadvantages in use. For example, known sensor-tipped IUP devices have an enlarged tip to accommodate the sensor located in the tip, and the enlarged tip often causes discomfort during insertion. Discomfort is also caused by the hardness of the material used to manufacture the tip. The major disadvantage of inserting sensor-tipped IUP devices, as well as fluid-filled IUP devices, is the possibility of perforating the placenta or uterus as a result of the higher insertion force required to insert the larger tip. More incidences of perforation are experienced with sensor-tipped IUP devices, and deaths have been reported of both fetuses and mothers from damage caused by insertion of sensor-tipped devices. A sensation of tingling at the site of the sensor has also been reported by some patients due to the electrical current which runs through the IUP device to the sensor in the tip.
Thus, it would be advantageous to the field of obstetrics to provide an intrauterine pressure monitor which is simple in construction, which is easily and quickly calibrated for accurately monitoring pressure changes, which is structured for easy and safe insertion into the uterus, which is structured to avoid damaging the uterus or endangering the fetus, which minimizes discomfort to the patient and which permits the patient to move freely without compromising the calibration or operation of the device.